Environmental and regulatory initiatives are requiring ever-lower levels of both sulfur and aromatics in transportation fuels. Sulfur limits for distillate fuels to be marketed in the European Union are already at 50 wppm, or less. There are also regulations that require lower levels of total aromatics in hydrocarbons and, more specifically, lower levels of multi-ring aromatics in transportation fuels, as well as for heavier hydrocarbon products. Further, the maximum allowable aromatics level for U.S. on-road diesel, California Air Resources Board (CARB) reference diesel, and Swedish Class I diesel are 35, 10 and 5 vol %, respectively. The CARB and Swedish Class I diesel fuel regulations allow no more than 1.4 and 0.02 vol % polynuclear aromatics, respectively. Consequently, much work is presently being done in the hydrotreating art because of these proposed regulations.
Hydrotreating, or in the case of sulfur removal, hydrodesulfurization, is well known in the art and typically requires treating a petroleum stream with hydrogen in the presence of a metal sulfide catalyst at hydrotreating conditions. The catalyst is typically comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support, such as alumina. Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten on alumina promoted with a metal such as cobalt, nickel, iron, or a combination thereof For example, cobalt promoted molybdenum on alumina catalysts are widely used for hydrodesulfurization. Nickel promoted molybdenum on alumina catalysts are also widely used for hydrodenitrogenation, partial aromatic saturation, as well as some hydrodesulfurization.
Attempts have been made to produce distillates having very low sulfur levels. Examples of these attempts include: the use of multi-stage processes that permit the separation of liquid and vapor between stages; the use of sulfur sensitive catalysts in a second stage; the use of alternative reactor designs in different stages; the hydrogenation or removal of aromatics; the preparation of various specialty products, other than low-sulfur products; and the use of a continuous excess supply of hydrogen. For example, Published United States Patent Application Number 2003/0168383 A1 teaches the use of at least five times the molar rate of chemical hydrogen consumption, but there is no definition of the term “chemical hydrogen consumption”. It would be extremely difficult to determine chemical hydrogen consumption “a priori” since one does not know how much hydrogen is used for aromatics hydrogenation, hydrodenitrogenation, and hydrocracking. Also, chemical hydrogen consumption would most likely be a function of hydrogen treat gas rate. The so-called “five-times” rule, if applicable, cannot be optimum over a wide range of hydrogen pressures. For example, in high-pressure operations, the so-called “five times” rule, may specify a treat gas rate (TGR) that is far in excess of what is needed to supply the catalyst surface with hydrogen. On the other hand, in low-pressure operations it may specify a TGR that is still short of what is needed to supply the desired surface hydrogen concentration. Moreover, the “five-times” rule does not consider other feedstock properties that are critical to desulfurization effectiveness.
Therefore, while various process scenarios have been developed for achieving deep desulfurization of petroleum distillate feedstocks, there still remains a need for ever improved, more efficient, and cost effective processes for achieving deep desulfurization of such feedstocks.